diff options
Diffstat (limited to 'Doc/using')
| -rw-r--r-- | Doc/using/cmdline.rst | 48 |
1 files changed, 47 insertions, 1 deletions
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst index 3fe0c7aedd..11e2d7d9c0 100644 --- a/Doc/using/cmdline.rst +++ b/Doc/using/cmdline.rst @@ -21,7 +21,7 @@ Command line When invoking Python, you may specify any of these options:: - python [-bBdEhiOsSuvVWx?] [-c command | -m module-name | script | - ] [args] + python [-bBdEhiORsSuvVWx?] [-c command | -m module-name | script | - ] [args] The most common use case is, of course, a simple invocation of a script:: @@ -215,6 +215,29 @@ Miscellaneous options Discard docstrings in addition to the :option:`-O` optimizations. +.. cmdoption:: -R + + Turn on hash randomization, so that the :meth:`__hash__` values of str, bytes + and datetime objects are "salted" with an unpredictable random value. + Although they remain constant within an individual Python process, they are + not predictable between repeated invocations of Python. + + This is intended to provide protection against a denial-of-service caused by + carefully-chosen inputs that exploit the worst case performance of a dict + insertion, O(n^2) complexity. See + http://www.ocert.org/advisories/ocert-2011-003.html for details. + + Changing hash values affects the order in which keys are retrieved from a + dict. Although Python has never made guarantees about this ordering (and it + typically varies between 32-bit and 64-bit builds), enough real-world code + implicitly relies on this non-guaranteed behavior that the randomization is + disabled by default. + + See also :envvar:`PYTHONHASHSEED`. + + .. versionadded:: 3.1.5 + + .. cmdoption:: -s Don't add user site directory to sys.path @@ -314,6 +337,7 @@ Miscellaneous options .. note:: The line numbers in error messages will be off by one. + Options you shouldn't use ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -328,6 +352,7 @@ Options you shouldn't use Reserved for alternative implementations of Python to use for their own purposes. + .. _using-on-envvars: Environment variables @@ -435,6 +460,27 @@ These environment variables influence Python's behavior. import of source modules. +.. envvar:: PYTHONHASHSEED + + If this variable is set to ``random``, the effect is the same as specifying + the :option:`-R` option: a random value is used to seed the hashes of str, + bytes and datetime objects. + + If :envvar:`PYTHONHASHSEED` is set to an integer value, it is used as a fixed + seed for generating the hash() of the types covered by the hash + randomization. + + Its purpose is to allow repeatable hashing, such as for selftests for the + interpreter itself, or to allow a cluster of python processes to share hash + values. + + The integer must be a decimal number in the range [0,4294967295]. Specifying + the value 0 will lead to the same hash values as when hash randomization is + disabled. + + .. versionadded:: 3.1.5 + + .. envvar:: PYTHONIOENCODING Overrides the encoding used for stdin/stdout/stderr, in the syntax |